Systems and methods for measuring sea-bed resistivity

ABSTRACT

A method for measuring the resistivity of sea-bed formations is described. An electromagnetic field is generated using at least one stationary long-range transmitter. The frequency of the electromagnetic field is between and/or including the ULF/ELF range. At least one component of the electromagnetic field is measured. A conductivity distribution is determined based on the at least one measured component. The determined conductivity distribution is correlated with geological formations and/or hydrocarbon deposits.

RELATED APPLICATIONS

This application is related to and claims priority from U.S. ProvisionalPatent Application Ser. No. 60/868,905 filed Dec. 6, 2006, for Systemsand Methods for Measuring Sea-Bed Resistivity, with inventors Evgeniy P.Velikhov and Michael S. Zhdanov, which is incorporated herein byreference.

TECHNICAL FIELD

The present invention relates generally to geophysical surveying. Morespecifically, the present invention relates to systems and methods formeasuring of sea-bed resistivity for off-shore hydrocarbon exploration.

BACKGROUND

Traditionally, oil and gas exploration is conducted using the seismicmethod. This geophysical technique provides a reasonable geometricalimage of the subsurface structures outlying the possible location andshape of the hydrocarbon deposit. However, the seismic method mayexperience difficulties in discriminating between the deposits filledwith water and the deposits filled with oil or gas. At the same time,the electrical properties of the water and hydrocarbon filled depositsmay differ dramatically because oil and gas generally have very highresistivity (up to about 10⁸ Ohm-m), while the water solutions in therock formations are typically very conductive (about 1 Ohm-m and below).

Many existing electromagnetic technologies for marine oil and gasexploration are generally based on using either the magnetotelluricmethods or placing the controlled source(s) in direct proximity to thetarget. There are very well known practical limitations of the marinecontrolled source electromagnetic (MCSEM) methods related to the limiteddepth of investigation. In order to increase the depth of theelectromagnetic field penetration, one should typically use largetransmitter/receiver offsets and, correspondingly, a very powerfultransmitter. Both of these requirements may increase the technologicaldifficulties as well as the cost of the MCSEM survey.

The magnetotelluric surveys are typically based on studying the electricand magnetic field variations at the sea-bottom due to the source in theionosphere/magnetosphere. The magnetotelluric field, because of itsregional nature, is practically uniform in the horizontal direction andmay generate relatively weak vertical currents. As a result, themagnetotelluric field generally has very limited sensitivity andresolution with respect to thin horizontal resistive targets that aretypical for the sea-bottom hydrocarbon deposits. In addition, thedownward propagating magnetotelluric field may attenuate rapidly withinthe conductive layer of the sea water, which may limit the practicalapplication of the magnetotelluric method in the deep-water surveys.Therefore a need exists for improved systems and methods for measuringsea-bed resistivity.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will become more fully apparentfrom the following description and appended claims, taken in conjunctionwith the accompanying drawings. Understanding that these drawings depictonly exemplary embodiments and are, therefore, not to be consideredlimiting of the invention's scope, the exemplary embodiments of theinvention will be described with additional specificity and detailthrough use of the accompanying drawings in which:

FIG. 1 is a conceptual block diagram illustrating an embodiment of asystem for measuring sea-bed resistivity;

FIG. 2 is a flow diagram of an embodiment of a method for measuringsea-bed resistivity;

FIG. 3 is a conceptual block diagram illustrating another embodiment ofa system for measuring sea-bed resistivity;

FIG. 4 is a conceptual block diagram illustrating a further embodimentof a system for measuring sea-bed resistivity;

FIG. 5 is a conceptual block diagram illustrating a still furtherembodiment of a system for measuring sea-bed resistivity;

FIG. 6 is a conceptual block diagram illustrating a further embodimentof a system for measuring sea-bed resistivity;

FIG. 7 is a flow diagram of another embodiment of a method for measuringsea-bed resistivity;

FIG. 8 is a flow diagram of a further embodiment of a method formeasuring sea-bed resistivity;

FIG. 9 is a flow diagram of a still further embodiment of a method formeasuring sea-bed resistivity;

FIG. 10 is a flow diagram of a further embodiment of a method formeasuring sea-bed resistivity; and

FIG. 11 illustrates various components that may be utilized in acomputing device.

DETAILED DESCRIPTION

A method of measuring the resistivity of sea-bed formations for mineralexploration with the goal for remote detection and imaging of thesea-bed hydrocarbon deposits, utilizing ULF/ELF electromagnetic signalsgenerated by the long range transmitter located on the land and/or inthe sea is described. Electromagnetic data may be acquired using anarray of electric (galvanic) and/or magnetic (induction) receiverslocated at the sea-bottom, and/or in the sea-bottom borehole to measureamplitude and phase of frequency or time domain responses in electricand magnetic fields of the ULF/ELF signals. The signals may be generatedby the remote transmitter located on the land and/or in the sea. Thecorresponding electromagnetic transfer functions may be determined fromthe observed ULF/ELF signals by using the linear relationships betweenthe different components of the electromagnetic fields. A 3Dconductivity distribution in the sea-bottom geological formation may bedetermined using a 3D electromagnetic inversion technique. The obtainedconductivity model may be correlated with known geological formationsfor sea-bed material characterization, remote detection and imaging ofthe sea-bed hydrocarbon deposits.

A method for measuring the resistivity of sea-bed formations isdescribed. An electromagnetic field is generated using at least onestationary long-range transmitter. The frequency of the electromagneticfield is between and/or including the ULF/ELF range. At least onecomponent of the electromagnetic field is measured. A conductivitydistribution is determined based on the at least one measured component.The determined conductivity distribution is correlated with geologicalformations and/or hydrocarbon deposits.

In some embodiments, generating an electromagnetic field includesgenerating the electromagnetic field over a plurality of frequencies. Inother embodiments, generating an electromagnetic field includesgenerating the electromagnetic field in the time domain. In furtherembodiments, generating the electromagnetic field in the time domainincludes using a magnetohydrodynamic (MHD) generator. In someembodiments, measuring at least one component of the electromagneticfield includes using a receiver that measures the electromagnetic fieldover a plurality of frequencies. In other embodiments, measuring atleast one component of the electromagnetic field includes using areceiver that measures the electromagnetic field in the time domain.

Generating an electromagnetic field, in some embodiments, includeslocating a transmitter on land or underwater. In further embodiments,the transmitter is an undersea communication cable.

In some embodiments, the electromagnetic field is generated by atransmitter formed by a system of grounded electric bipoles of severalkilometers in length or formed by a loop of wire with the radius ofseveral kilometers. For example, the length and/or radius of thetransmitter may be more than about three kilometers. In furtherembodiments, measuring at least one component of the electromagneticfield further comprises using at least one galvanic receiver and/or atleast one induction receiver. In still further embodiments, the voltagedetected in at least one of the receivers is recorded.

Measuring at least one component of the electromagnetic field, in someembodiments, includes measuring the amplitude and/or phase of theelectromagnetic field. In further embodiments, measuring at least onecomponent of the electromagnetic field includes using at least onereceiver located on a sea-bed and/or at least one receiver located in asea-bed borehole.

In some embodiments, measuring at least one component of theelectromagnetic field includes using at least one moving receiverlocated underwater. In further embodiments, measuring at least onecomponent of the electromagnetic field includes using a receiver thatmeasures a magnetic component and/or electric component of theelectromagnetic field.

Determining a conductivity distribution, in some embodiments, is basedon a plurality of measured components of the electromagnetic field andthe conductivity distribution is determined by determining at least onetransfer function using the plurality of measured components of theelectromagnetic field. In further embodiments, determining at least onetransfer function includes determining at least one electric transferfunction, magnetic transfer function, impedance transfer function and/oradmittance transfer function.

In some embodiments, determining at least one transfer function includesusing a linear relationship between a first component of theelectromagnetic field and a second component of the electromagneticfield. In further embodiments, determining at least one transferfunction includes using a least-squares method.

Correlating the determined conductivity distribution with geologicalformations and/or hydrocarbon deposits, in some embodiments, includescharacterizing sea-bed material, remotely detecting sea-bed hydrocarbondeposits and/or imaging the sea-bed hydrocarbon deposits.

In some embodiments, correlating the determined conductivitydistribution with geological formations and/or hydrocarbon depositsincludes determining a three-dimensional conductivity distribution. Infurther embodiments, determining a three-dimensional conductivitydistribution includes using a three-dimensional inversion technique. Instill further embodiments, the three-dimensional inversion technique isbased on a regularized three-dimensional focusing nonlinear inversion ofthe at least one measured component of the electromagnetic field.

Correlating the determined conductivity distribution with geologicalformations and/or hydrocarbon deposits, in some embodiments, includescomparing observed data with predicted data. In further embodiments,comparing observed data with predicted data includes minimizing aparametric functional. In still further embodiments, minimizing aparametric functional includes using gradient type methods and/or amisfit functional and a stabilizer.

In some embodiments, correlating the determined conductivitydistribution with geological formations and/or hydrocarbon depositsincludes stacking the measured at least one component of theelectromagnetic field with a corresponding at least one component of theelectromagnetic field measured at another period.

Another embodiment of a method for locating hydrocarbon deposits isdescribed. An electromagnetic field is generated using at least onestationary long-range transmitter. The frequency of the electromagneticfield is between and/or including the ULF/ELF range. At least onecomponent of the electromagnetic field is measured using a plurality ofreceivers. At least one transfer function is determined based on the atleast one measured component. The determined transfer function iscorrelated with geological formations and/or hydrocarbon deposits.

A further embodiment of a method for locating hydrocarbon deposits isdescribed. An electromagnetic field is generated using at least oneundersea communication cable carrying a frequency domain current. Thefrequency of the electromagnetic field is within the ULF/ELF range. Aplurality of components of the electromagnetic field are measured usinga plurality of magnetic and/or electric receivers that are located atthe sea-bottom, wherein the plurality of receivers measure theelectromagnetic field over a plurality of frequencies. At least onetransfer function is determined based on the plurality of measuredcomponents. At least one of the following transfer functions is anelectric transfer function, a magnetic transfer function, an impedancetransfer function and/or an admittance transfer function. The determinedat least one transfer function is correlated with geological formationsand/or hydrocarbon deposits by determining a three-dimensionalconductivity distribution using a three-dimensional inversion techniquebased on a regularized three-dimensional focusing nonlinear inversion ofthe plurality of measured components of the electromagnetic field.Observed data is compared with predicted data by minimizing a parametricfunctional using gradient type methods and/or a misfit functional and astabilizer. The plurality of measured components of the electromagneticfield are stacked with a corresponding plurality of measured componentsof the electromagnetic field measured at another period.

Various embodiments of the invention are now described with reference tothe Figures. The embodiments of the present invention, as generallydescribed and illustrated in the Figures herein, could be arranged anddesigned in a wide variety of different configurations. Thus, thefollowing more detailed description of several exemplary embodiments ofthe present invention, as represented in the Figures, is not intended tolimit the scope of the invention, as claimed, but is merelyrepresentative of the embodiments of the invention.

The word “exemplary” is used exclusively herein to mean “serving as anexample, instance, or illustration.” Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments.

Many features of the embodiments disclosed herein may be implemented ascomputer software, electronic hardware or combinations of both. Toclearly illustrate this interchangeability of hardware and software,various components will be described generally in terms of theirfunctionality. Whether such functionality is implemented as hardware orsoftware depends upon the particular application and design constraintsimposed on the overall system. Skilled artisans may implement thedescribed functionality in varying ways for each particular application,but such implementation decisions should not be interpreted as causing adeparture from the scope of the present invention.

Where the described functionality is implemented as computer software,such software may include any type of computer instruction or computerexecutable code located within a memory device and/or transmitted aselectronic signals over a system bus or network. Software thatimplements the functionality associated with components described hereinmay comprise a single instruction, or many instructions, and may bedistributed over several different code segments, among differentprograms, and across several memory devices.

As used herein, the terms “an embodiment,” “embodiment,” “embodiments,”“the embodiment,” “the embodiments,” “one or more embodiments,” “someembodiments,” “certain embodiments,” “one embodiment,” “anotherembodiment” and the like mean “one or more (but not necessarily all)embodiments of the disclosed invention(s),” unless expressly specifiedotherwise.

The term “determining” (and grammatical variants thereof) is used in anextremely broad sense. The term “determining” encompasses a wide varietyof actions and therefore “determining” can include calculating,computing, processing, deriving, investigating, looking up (e.g.,looking up in a table, a database or another data structure),ascertaining and the like. Also, “determining” can include receiving(e.g., receiving information), accessing (e.g., accessing data in amemory) and the like. Also, “determining” can include resolving,selecting, choosing, establishing and the like.

The phrase “based on” does not mean “based only on”, unless expresslyspecified otherwise. In other words, the phrase “based on” describesboth “based only on” and “based at least on.”

It would be an advantage over existing magnetotelluric and marinecontrolled source electromagnetic (MCSEM) techniques to provide a systemand a method for direct deposit imaging and quantitative evaluation ofits geoelectrical parameters utilizing the ultra low and extremely lowfrequency (ULF and ELF) radio communication signal in the range of about0.01 Hz to about 30 Hz, which may penetrate deep enough to reach ahydrocarbon deposit. ULF and ELF signals may provide nearly worldwidecoverage, which make them an attractive and reliable source forpractical geophysical exploration. The ULF/ELF radio communicationsystem may also be characterized by an extremely narrow and stablefrequency range with a very narrow frequency shift, which may allow forstacking the recorded signal to improve the signal-to-noise ratio.

Undersea communication cables may represent another type of lowfrequency transmitting system. The network of submarine cables generallycovers wide areas of the world ocean providing a practically free sourceof transmitting electromagnetic energy, which can be efficiently usedfor geophysical exploration as well.

The systems and methods disclosed herein may be used for sea-bedresistivity imaging of the hydrocarbon deposits using the ULF/ELFelectromagnetic signals of a remote powerful electromagneticcommunication transmitter, which may be located on land and/or at thesea-bottom. A geophysical method of hydrocarbon exploration usingundersea communication cables as a source of the ULF/ELF electromagneticsignals is also described. A method and numerical scheme forquantitative interpretation of the ULF/ELF field measured within thesea-water layer and at the sea-bottom is described. Although theexamples provided herein are generally directed to sea-bed resistivityimaging of hydrocarbon deposits, other uses are also contemplated.Furthermore, although detection of hydrocarbon deposits is described,mapping of areas where hydrocarbon deposits were not detected, acombination of the two and/or other uses may also be contemplated.

Systems and methods for measuring the resistivity of the sea-bedformations and imaging a sea-bed hydrocarbon deposit using the ULF/ELFsignal of the remote electromagnetic transmitter is provided. Themethods may include measuring the magnetic and electric fields generatedby a stationary transmitter operating in the ULF/ELF range (on the orderof about 0.01 Hz to about 30 Hz). A stationary transmitter may belocated on the land or at the sea-bottom. The measurements may beconducted by an array of receivers located at some depth within thesea-water layer, and/or at the sea-bottom, and/or in the sea-bottomborehole at a large distance (for example, from about ten kilometers upto about a thousand kilometers) from the transmitter.

The transmitter location may be selected either on the land in the areawith the outcropping resistive earth crust basement or directly at thesea-bottom, in order to ensure that the electromagnetic field generatedby the transmitter propagates along two propagation paths of lowfrequency waves: the first path may be formed by the earth-ionosphericwave guide and the second path may be represented by the undergroundwave guide formed by the resistive thickness of the earth crust. As aresult, the resistivity distribution of earth formations penetrated bythe ULF/ELF electromagnetic field may be determined by taking intoaccount both the electromagnetic signals arriving at the receivers bythe earth-ionospheric wave guide and by the underground wave guideformed by the resistive thickness of the earth crust. Therefore, themethod may be used even in the deep-water settings, where both thenatural magnetotelluric field and the part of the ULF/ELF signalpropagating from the ionosphere typically cannot penetrate through thethick conductive layer of the sea water.

The systems and methods may be used for direct deposit imaging andquantitative evaluation of its geoelectrical parameters utilizing theultra low and extremely low frequency (ULF and ELF) electromagneticsignals in the range of about 0.01 Hz to about 30 Hz, which maypenetrate deep enough within the sea-bottom formations to reach thehydrocarbon deposit. In the present embodiment, signals may range infrequency from about 0.01 Hz to about 30 Hz.

In one embodiment of the invention the measurements may be conducted byan array of fixed electric and/or magnetic receivers located at thesea-bottom. One receiver position may be selected as a referenceposition (the reference station). The corresponding transfer functionsbetween the electromagnetic data in the reference station position andin the array of receivers may be calculated. These transfer functionsmay be independent of the configuration and location of the transmitterand they may only depend on the resistivity distribution in thegeological formations. The quantitative interpretation of the observeddata and imaging of the sea-bottom hydrocarbon deposit may be based onthe analysis of the corresponding electromagnetic transfer functions.

In an alternative embodiment of the invention, the measurements may beconducted by the sets of moving and fixed electric and/or magneticreceivers and the corresponding transfer functions may be calculatedbetween the moving set of the receivers and the fixed set of thereceivers.

In another embodiment of the invention, the frequency domain current inthe transmitter may be generated for at least several frequencies, andthe receivers may measure the signal at several frequencies. Themulti-frequency measurements may be used for frequency electromagneticsounding of the medium at the receiver locations, using both theelectromagnetic signals arriving at the receivers by theearth-ionospheric wave guide and by the underground wave guide formed bythe resistive thickness of the earth crust.

Yet another embodiment of the invention may provide a newelectromagnetic system and method for determining the resistivity imageof a sea-bed hydrocarbon deposit using the ULF/ELF signal of the remoteelectromagnetic transmitter in the time domain. The time domainmeasurements may be used for transient electromagnetic sounding of themedium at the receiver locations, using both the electromagnetic signalsarriving at the receivers by the earth-ionospheric wave guide and byusing the underground wave guide formed by the resistive thickness ofthe earth crust.

In another embodiment, the time domain current in the transmitterlocated on the land or at the sea-bottom may be generated with thepowerful pulse magneto hydrodynamic (MHD) generator.

The electromagnetic field generated by remote transmitter at the ultralow or extremely low (ULF/ELF) frequency may be utilized for the sea-bedresistivity imaging of the off-shore hydrocarbon deposits.

In the illustrated embodiments, two major types of transmittingfacilities may be used generally in order to effectively utilize theelectromagnetic field propagating to the sea-bottom receivers throughthe underground wave guide formed by the resistive thickness of theearth crust: (1) a stationary transmitter may be located on the land,preferably, in the area with the outcropping resistive earth crustbasement and the electromagnetic transmitter may be formed by a systemof grounded electric bipoles with several kilometers length (up to about100 kilometers), and/or by a horizontal electric loop of wire with theradius of several kilometers, and (2) a stationary transmitter may belocated at the sea-bottom and may be formed by a long underseacommunication cable carrying a frequency domain electric current. Inother embodiments, other types of transmitting facilities may be used toutilize the electromagnetic field propagating through the undergroundwave guide formed by the earth's crust.

Some embodiments may provide a new capability for determining theresistivity distribution of earth formations penetrated by the ULF/ELFelectromagnetic field by taking into account both the electromagneticsignals arriving at the receivers by the earth-ionospheric wave guideand by the underground wave guide formed by the resistive thickness ofthe earth's crust. In order to enhance this capability, the transmittingfacility on the land may be formed by the grounded electric bipole(s)sending the current into the earth's formations. In the case of theundersea electric cable transmitter, the main part of the generatedelectromagnetic field may also propagate through the underseaformations, especially in the deep ocean areas.

FIG. 1 is a conceptual block diagram illustrating an embodiment of asystem 100 for measuring sea-bed resistivity. The system 100 may includean electromagnetic transmitter 102 and a receiver 104. Theelectromagnetic transmitter 102 may generate an electromagnetic field.

The generated electromagnetic field may be propagated through theearth-ionospheric wave guide 106 and/or the underground wave guide 108formed within the earth's crust. The generated electromagnetic field mayreach the receiver 104. The receiver 104 may be used to measure at leastone component of the electromagnetic field.

A hydrocarbon deposit 110 may be located within the sea-bed 112. Forexample, the sea-bed 112 may include geological formations with a knownconductivity distribution.

FIG. 2 is a flow diagram of an embodiment of a method 200 for measuringsea-bed resistivity. The method 200 may include generating 202 anelectromagnetic field. The electromagnetic field may be generated 202 byan electromagnetic transmitter 102.

At least one component of the electromagnetic field may be measured 204.At least one receiver 104 may measure 204 the electromagnetic field. Forexample, at least one receiver 104 may measure 204 at least onecomponent of the electromagnetic field. In some embodiments, theamplitude and/or phase of the electromagnetic field may be measured 204.In further embodiments, the magnetic and/or electric components of theelectromagnetic field may be measured 204.

A conductivity distribution may be determined 206 based on the at leastone measured component. The conductivity distribution may include theconductivity distribution for a hydrocarbon deposit 110, geologicalformations, and/or other formations and/or deposits.

The determined conductivity distribution may be correlated 208 withgeological formations and/or hydrocarbon deposits 110.

FIG. 3 is a conceptual block diagram illustrating another embodiment ofa system 300 for measuring sea-bed resistivity. The system 300 mayinclude an electromagnetic transmitter 302 and at least one receiver304.

The electromagnetic transmitter 302 may be located on land 314. Theelectromagnetic transmitter 302 may be a stationary long-rangetransmitter. In the present embodiment, the electromagnetic transmitter302 may be a system of grounded electric bipoles. The system of groundedelectric bipoles may be more than three kilometers in length. In otherembodiments, the electromagnetic transmitter 302 may be a loop of wire.The loop of wire may have a radius of more than three kilometers.

The at least one receiver 304 may be located in the seawater 316. Inother embodiments, the receivers 304 may be located in freshwater, etc.The receivers 304 may be located on a sea-bed 112. In other embodiments,the receivers 304 may be located in a borehole. For example, thereceivers 304 may be located in a borehole in the sea-bed 112. In thepresent embodiment, the receivers 304 may be stationary.

In the present embodiment, three receivers 304 may be used. In otherembodiments, more or fewer receivers 304 may be used. In someembodiments, the system 300 may be a ULF/ELF sea-bed electromagnetic(USBEM) survey configuration using one on land electromagnetictransmitter 302 and an array of fixed sea-bottom receivers 304 ofelectric and/or magnetic fields.

The electromagnetic transmitter 302 may generate an electromagneticfield. In the present embodiment, the frequency of the electromagneticfield may be between and/or including the ELF and ULF range. In otherembodiments, the frequency of the electromagnetic field may be inanother range. For example, the frequency may be in the ELF, SLF, ULFand/or another frequency range. The frequency range may be selectedbased on whether the frequency range may propagate through theearth-ionospheric wave guide 106 and/or the underground wave guide 108formed within the earth's crust.

The generated electromagnetic field may be propagated through theearth-ionospheric wave guide 106 and/or the underground wave guide 108formed within the earth's crust. The generated electromagnetic field mayreach the receivers 304.

The receivers 304 may be used to measure at least one component of theelectromagnetic field. The receivers 304 may be galvanic, inductionand/or other receiver types. At least one receiver 304 may recordvoltage detected by the at least one receiver 304. The receivers 304 maymeasure at least one component of the electromagnetic field. Forexample, the receivers 304 may measure the amplitude and/or phase of theelectromagnetic field. In another example, the receivers 304 may measuremagnetic and/or electric components of the electromagnetic field.

A hydrocarbon deposit 110 may be located within the sea-bed 112. Forexample, the sea-bed 112 may include geological formations with a knownconductivity distribution.

FIG. 4 is a conceptual block diagram illustrating a further embodimentof a system 400 for measuring sea-bed resistivity. The system 400 mayinclude an electromagnetic transmitter 402 and at least one receiver304.

The electromagnetic transmitter 402 may be located in the seawater 316.The electromagnetic transmitter 402 may be an undersea communicationcable. The electromagnetic transmitter 402 may be located on a sea-bed112. In the present embodiment, the electromagnetic transmitter 402 islocated at the sea bottom.

The at least one receiver 304 may be located in the seawater 316. Inother embodiments, the receivers 304 may be located in freshwater, etc.The receivers 304 may be located on a sea-bed 112. In other embodiments,the receivers 304 may be located in a borehole. For example, thereceivers 304 may be located in a borehole at the sea bottom.

In the present embodiment, three receivers 304 may be used. In otherembodiments, more or fewer receivers 304 may be used. In someembodiments, the system 400 may be a ULF/ELF sea-bed electromagnetic(USBEM) survey configuration using a submarine cable electromagnetictransmitter 402 and an array of fixed sea-bottom receivers 304 ofelectric and/or magnetic fields.

The electromagnetic transmitter 402 may generate an electromagneticfield. In the present embodiment, the frequency of the electromagneticfield may be between and/or including the ELF and ULF range. In otherembodiments, the frequency of the electromagnetic field may be inanother range. For example, the frequency may be in the ELF, SLF, ULFand/or another frequency range. The frequency range may be selectedbased on whether the frequency range may propagate through theearth-ionospheric wave guide 106 and/or the underground wave guide 108formed within the earth's crust.

The generated electromagnetic field may be propagated through theearth-ionospheric wave guide 106 and/or the underground wave guide 108formed within the earth's crust. The generated electromagnetic field mayreach the receivers 304.

The receivers 304 may be used to measure at least one component of theelectromagnetic field. The receivers 304 may be galvanic, inductionand/or other receiver types. At least one receiver 304 may recordvoltage detected by the at least one receiver 304. The receivers 304 maymeasure at least one component of the electromagnetic field. Forexample, the receivers 304 may measure the amplitude and/or phase of theelectromagnetic field. In another example, the receivers 304 may measurea magnetic, electric and/or other component of the electromagneticfield.

A hydrocarbon deposit 110 may be located within the sea-bed 112. Forexample, the sea-bed 112 may include geological formations with a knownconductivity distribution.

FIG. 5 is a conceptual block diagram illustrating a still furtherembodiment of a system 500 for measuring sea-bed resistivity. The system500 may include an electromagnetic transmitter 302 and at least onereceiver 304.

The electromagnetic transmitter 302 may be located on land 314. Theelectromagnetic transmitter 302 may be a stationary long-rangetransmitter. In the present embodiment, the electromagnetic transmitter302 may be a system of grounded electric bipoles. The system of groundedelectric bipoles may be several kilometers in length. For example, thesystem of grounded bipoles may be more than three kilometers in length.In other embodiments, the electromagnetic transmitter 302 may be a loopof wire. The loop of wire may have a radius of more than severalkilometers. For example, the loop of wire may have a radius of more thanthree kilometers.

The receivers 304 may be located in the seawater 316. In otherembodiments, the receivers 304 may be located in freshwater, etc. Thereceivers 304 may be located on a sea-bed 112. In other embodiments, thereceivers 304 may be located in a borehole. For example, the receivers304 may be located in a borehole at the sea bottom. In the presentembodiment, the system 400 may include at least one receiver 304 locatedon a sea-bed 112 and/or at least one moving receiver 504. The at leastone moving receiver 504 may be towed by a survey vessel 518. In thepresent embodiment, only one moving receiver 504 is illustrated. Inother embodiments, multiple moving receivers 504 may be used. In someembodiments, the system 500 may be a USBEM survey configuration using anon land electromagnetic transmitter 302, a set of fixed sea-bottomreceivers 304, and another set of moving receivers 504 of electricand/or magnetic fields, towed by a survey vessel 518.

The electromagnetic transmitter 302 may generate an electromagneticfield. In the present embodiment, the frequency of the electromagneticfield may be between and/or including the ELF and ULF range. In otherembodiments, the frequency of the electromagnetic field may be inanother range. For example, the frequency may be in the ELF, SLF, ULFand/or another frequency range. The frequency range may be selectedbased on whether the frequency range may propagate through theearth-ionospheric wave guide 106 and/or the underground wave guide 108formed within the earth's crust.

The generated electromagnetic field may be propagated through theearth-ionospheric wave guide 106 and/or the underground wave guide 108formed within the earth's crust. The generated electromagnetic field mayreach the receivers 304, 504.

The receivers 304, 504 may be used to measure at least one component ofthe electromagnetic field. The receivers 304, 504 may be galvanic,induction and/or other receiver types. At least one receiver 304, 504may record voltage detected by the at least one receiver 304, 504. Thereceivers 304, 504 may measure at least one component of theelectromagnetic field. For example, the receivers 304, 504 may measurethe amplitude and/or phase of the electromagnetic field. In anotherexample, the receivers 304, 504 may measure a magnetic, electric and/orother component of the electromagnetic field.

A hydrocarbon deposit 110 may be located within the sea-bed 112. Forexample, the sea-bed 112 may include geological formations with a knownconductivity distribution.

FIG. 6 is a conceptual block diagram illustrating a further embodimentof a system 600 for measuring sea-bed resistivity. The system 600 mayinclude an electromagnetic transmitter 402 and at least one receiver304, 504.

The electromagnetic transmitter 402 may be located in the seawater 316.The electromagnetic transmitter 402 may be an undersea communicationcable. The electromagnetic transmitter 402 may be located on a sea-bed112. In the present embodiment, the electromagnetic transmitter 402 islocated at the sea bottom.

The receivers 304, 504 may be located in the seawater 316. In otherembodiments, the receivers 304, 504 may be located in freshwater, etc.Some of the receivers 304 may be located on a sea-bed 112. In otherembodiments, some of the receivers 304 may be located in a borehole. Forexample, the receivers 304 may be located in a borehole in the sea-bed112. In the present embodiment, the system 400 may include at least onereceiver 304 located on a sea-bed 112 and/or at least one movingreceiver 504. The at least one moving receiver 504 may be towed by asurvey vessel 518. In the present embodiment, only one moving receiver504 is illustrated. In other embodiments, multiple moving receivers 504may be used. In some embodiments, the system 600 may be a USBEM surveyconfiguration using a submarine cable electromagnetic transmitter 402, aset of fixed sea-bottom receivers 304, and another set of movingreceivers 504 of electric and/or magnetic fields, towed by the surveyvessel 518.

The electromagnetic transmitter 402 may generate an electromagneticfield. In the present embodiment, the frequency of the electromagneticfield may be between and/or including the ELF and ULF range. In otherembodiments, the frequency of the electromagnetic field may be inanother range. For example, the frequency may be in the ELF, SLF, ULFand/or another frequency range. The frequency range may be selectedbased on whether the frequency range may propagate through theearth-ionospheric wave guide 106 and/or the underground wave guide 108formed within the earth's crust.

The generated electromagnetic field may be propagated through theearth-ionospheric wave guide 106 and/or the underground wave guide 108formed within the earth's crust. The generated electromagnetic field mayreach the receivers 304, 504.

The receivers 304, 504 may be used to measure at least one component ofthe electromagnetic field. The receivers 304, 504 may be galvanic,induction and/or other receiver types. At least one receiver 304, 504may record voltage detected by the at least one receiver 304, 504. Thereceivers 304, 504 may measure at least one component of theelectromagnetic field. For example, the receivers 304, 504 may measurethe amplitude and/or phase of the electromagnetic field. In anotherexample, the receivers 304, 504 may measure a magnetic, electric and/orother component of the electromagnetic field.

A hydrocarbon deposit 110 may be located within the sea-bed 112. Forexample, the sea-bed 112 may include geological formations with a knownconductivity distribution.

FIG. 7 is a flow diagram of an embodiment of a method 700 for measuringsea-bed resistivity. The method 700 may include generating 702 anelectromagnetic field. The electromagnetic field may be generated 702 byan electromagnetic transmitter 102. The electromagnetic transmitter 102may be located on land 314, like the electromagnetic transmitter 302described in FIGS. 3 and 5, or underwater, like the electromagnetictransmitter 402 described in FIGS. 4 and 6.

A plurality of components of the electromagnetic field may be measured704. Receivers 104 may measure 704 the plurality of components of theelectromagnetic field. In some embodiments, one receiver 104 may be usedto measure 704 a plurality of components of the electromagnetic field.For example, one receiver 104 may measure 704 the x and the y componentof the electrical field. In other embodiments, a plurality of receivers104 may be used to measure 704 a plurality of components of theelectromagnetic field. For example, two receivers 104 may measure 704the x component of the magnetic field. Other directional components ofthe electric and/or magnetic portion of the electromagnetic field may bemeasured 704 by one or more receivers 104.

At least one transfer function may be determined 706. The transferfunction may be determined based on the plurality of measured componentsof the electromagnetic field. The determined at least one transferfunction may be correlated 708 with geological formations and/orhydrocarbon deposits 110.

For example, at least one receiver 104 may be located at a point with aradius vector r₀ of some Cartesian coordinates and at least one otherreceiver 104 may be located at a point with a variable radius vector r.The receivers 104 may measure 704 any combination of the components ofthe electromagnetic field: {E_(x), E_(y), E_(z), H_(x), H_(y), H_(z)}.The electromagnetic field components observed in point r are linearlyproportional to the electromagnetic field components observed in thereference point r₀

$\begin{matrix}{{{E_{\alpha}(r)} = {\sum\limits_{{\beta = x},y,z}{{T_{\alpha\beta}\left( {r,r_{0}} \right)}\mspace{11mu}{E_{\beta}\left( r_{0} \right)}}}},} & (1) \\{{{H_{\alpha}(r)} = {\sum\limits_{{\beta = x},y,z}{{M_{\alpha\beta}\left( {r,r_{0}} \right)}\mspace{11mu}{H_{\beta}\left( r_{0} \right)}}}},} & (2) \\{{{E_{\alpha}(r)} = {\sum\limits_{{\beta = x},y,z}{{Z_{\alpha\beta}\left( {r,r_{0}} \right)}\mspace{11mu}{H_{\beta}\left( r_{0} \right)}}}},} & (3) \\{{{H_{\alpha}(r)} = {\sum\limits_{{\beta = x},y,z}{{Y_{\alpha\beta}\left( {r,r_{0}} \right)}\mspace{11mu}{E_{\beta}\left( r_{0} \right)}}}},\mspace{14mu}{\alpha = x},y,z,} & (4)\end{matrix}$

where T_(αβ), M_(αβ), Z_(αβ), and Y_(αβ) are scalar electromagnetictransfer functions. T_(αβ) and M_(αβ) are electric and magnetic transferfunctions, while Z_(αβ) and Y_(αβ) are impedance and admittance transferfunctions, respectively. In the present embodiment, at least onetransfer function may be determined 706 based on the plurality ofmeasured components of the electromagnetic field.

The transfer functions T_(αβ), M_(αβ), Z_(αβ), and Y_(αβ) depend on thecoordinates of the observation points, r and r₀, the frequency, ω,and/or the distribution of electrical conductivity in the medium, σ(r).However, the transfer functions T_(αβ), M_(αβ), Z_(αβ), and Y_(αβ) areindependent of the strength and configuration of the current in thetransmitter 102. The transfer functions T_(αβ), M_(αβ), Z_(αβ), andY_(αβ), in contrast to measured electric and magnetic fields, carryinformation about the internal geoelectrical structure of the earthonly. In other embodiments, other transfer functions may includetransfer functions other than the T_(αβ), M_(αβ), Z_(αβ), and Y_(αβ)transfer functions.

For example, in the embodiment illustrated in FIG. 5, the stationaryreceivers 304 may be located at a point r₀, and the moving receivers 504may be located at a point with the radius vector r. The stationaryreceivers 304 may measure any combination of the components of theelectromagnetic field {E_(x), E_(y), E_(z), H_(x), H_(y), H_(z)}, themoving receivers 504 may measure any combination of the components ofthe electromagnetic field, for example, electric field component E_(y).In this case, i.e. when measuring all components of the electromagneticfield, the six fields T_(yβ) (r, r₀) and Z_(yβ) (r, r₀) representing theelectric and impedance transfer functions along the survey profile orover the survey area may be determined 706.

In another example, in the embodiment illustrated in FIG. 6, thestationary receivers 304 may be located at a point r₀, and the movingreceivers 504 may be located at a point with the radius vector r. Thestationary receivers 304 may measure any combination of theelectromagnetic field components {E_(x), E_(y), E_(z), H_(z), H_(y),H_(z)}; the moving receivers 504 may also measure any combination of theelectromagnetic field components, for example, electric field componentE_(y).

The least squares method may be used to determine 706 at least onetransfer function. For example, let us assume that we have a series ofmeasurements of the ULF/ELF signal at a given frequency, ω,E_(i) ^(x),E_(i) ^(y),E_(i) ^(z),H_(i) ^(x),H_(i) ^(y),H_(i) ^(z)i=1, 2,. . . , N.

Consider, as an example, the electric transfer function, T_(αβ).

In accord with equations (1), we write:

$\begin{matrix}{{{E_{\alpha\; i}(r)} = {{\sum\limits_{{\beta = x},y,z}{{T_{\alpha\beta}\left( {r,r_{0}} \right)}\mspace{11mu}{E_{\beta\; i}\left( r_{0} \right)}}} + e_{\alpha\; i}}},{\alpha = x},y,{z;\;{i = 1}},2,\ldots\mspace{11mu},{N;}} & (5)\end{matrix}$

where e_(αi) may be error terms, caused by the noise in the data.

The least squares method may permit us to find the transfer functions,which may minimize the weighted sum of the squares of the absolutevalues of the errors in the linear relationship:

$\begin{matrix}\begin{matrix}{{\phi\;\left( T_{\alpha\beta} \right)} = {\sum\limits_{{\alpha = x},y,z}{\sum\limits_{i = 1}^{N}{w_{\alpha\; i}{e_{\alpha\; i}}^{2}}}}} \\{= {\sum\limits_{{\alpha = x},y,z}{\sum\limits_{i = 1}^{N}{w_{\alpha\; i}{{{E_{\alpha\; i}(r)} - {{T_{\alpha\beta}\left( {r,r_{0}} \right)}\;{E_{\beta\; i}\left( r_{0} \right)}}}}^{2}}}}} \\{{= \min},}\end{matrix} & (6)\end{matrix}$

where weights, w_(αi), may be inversely proportional to the dispersionsof the errors:w _(αi)=1/σ² _(αi).  (7)

The variational operator may be applied with respect to the transferfunctions to functional φ and the result may be equaled to zero toobtain a system of linear equations for T_(αβ) which have the followingsolution:T _(αβ)(r,r ₀)=S _(αβ)(r,r ₀)/S _(αβ)(r ₀),  (8)

where

$\begin{matrix}\begin{matrix}{{{S_{\alpha\beta}\left( {r,r_{0}} \right)} = {\sum\limits_{i = 1}^{N}{w_{\alpha\; i}{E_{\alpha\; i}(r)}\mspace{11mu}{E_{\beta\; i}^{*}\left( r_{0} \right)}}}},} \\{= {\sum\limits_{i = 1}^{N}{w_{\alpha\; i}{{{E_{\beta\; i}^{*}\left( r_{0} \right)}}^{2}.}}}}\end{matrix} & (9)\end{matrix}$

In summary, the least squares method with weights may allow us toexclude or reduce the effect of the errors with unequal dispersions onthe results of the transfer functions calculations from the ULF/ELFdata. In other embodiments, other methods may be used to determine 706the at least one transfer function.

The determined at least one transfer function may be correlated 708 withgeological formations and/or hydrocarbon deposits 110. The determined atleast one transfer function may be correlated 708 with geologicalformations and/or hydrocarbon deposits 110 may be used to determine thelocation of a hydrocarbon deposit 110.

Correlating 708 the determined at least one transfer function may beaccomplished using the following exemplary steps. For example, we may,generally, consider an appropriate geoelectrical model of the sea-bottomgeological formation. The interpretation problem may be formulated forthe USBEM data measured 704 at the sea-bottom.

The field measured 704 by the receivers 104 may be represented as a sumof the background electromagnetic field, {E^(b), H^(b)}, which may begenerated in the background model formed by the sea water and thesedimental layers, and an anomalous part, {E^(a), H^(a)} related to theanomalous conductivity Δσ (the conductivity inhomogeneities) present inthe sea-bottom:E=E ^(b) +E ^(a) ,H=H ^(b) +H ^(a).

We may use the integral form of Maxwell's equations to express theelectromagnetic field measured 704 by the receivers 104:

$\begin{matrix}{{{E_{\alpha}\left( r_{j} \right)} = {{\sum\limits_{{\beta = x},y,z}{\int{\int{\int_{D}{{{G_{E\;\alpha\;\beta}\left( r_{j} \middle| r \right)} \cdot \left\lbrack {\Delta\;\sigma\mspace{11mu}(r)\mspace{11mu}{E_{\beta}(r)}} \right\rbrack}\ {\mathbb{d}v}}}}}} + {E_{\alpha}^{b}\left( r_{j} \right)}}},} & (10)\end{matrix}$

$\begin{matrix}{{{H_{\alpha}\left( r_{j} \right)} = {{\sum\limits_{{\beta = x},y,z}{\int{\int{\int_{D}{{{G_{H\;\alpha\;\beta}\left( r_{j} \middle| r \right)} \cdot \left\lbrack {\Delta\;\sigma\mspace{11mu}(r)\mspace{11mu}{E_{\beta}(r)}} \right\rbrack}\ {\mathbb{d}v}}}}}} + {H_{\alpha}^{b}\left( r_{j} \right)}}},{\alpha = x},y,z,} & (11)\end{matrix}$

where G_(Eαβ)(r_(j)|r) and G_(Hαβ)(r_(j)r) (α,β=x, y, z) are thecomponents of the electric and magnetic Green's tensors defined for amedium where the background conductivity σ_(b) and domain D mayrepresent a volume with the anomalous conductivity distributionσ(r)=σ_(b)+Δσ(r), rεD.

Substituting equations (10) and (11) into expressions (1)-(4) andsolving the last equations with respect to T_(αβ), M_(αβ), Z_(αβ), andY_(αβ) we may determine 706 the corresponding transfer functions.

In short form, the relationships between the anomalous conductivity, Δσand the transfer functions, T_(αβ), M_(αβ), Z_(αβ), and Y_(αβ) expressedby equations (10)-(11) and expressions (1)-(4) may be correlated 708 asan operator equation:d=A(Δσ),  (12)

where A may be a forward modeling operator, d may stand for thecorresponding transfer functions computed from the observed (i.e.measured 704) electromagnetic data in the sea-bottom receivers, and Δσmay be a vector formed by the anomalous conductivities within thetargeted domain.

Note that a sea-water layer may usually be characterized by a lowresistivity of about 0.25 Ohm-m, and the sea-bottom sediments may alsobe very conductive with the resistivity of the order of 1 Ohm-m. At thesame time, the sea-bottom hydrocarbon deposits 110 may usually becharacterized by relatively high resistivity in the range from tens ofOhm-m up to several hundred Ohm-m. Therefore, the hydrocarbon deposit110 may represent a relatively strong resistivity anomaly with thenegative anomalous conductivity, Δσ. The correlation 708 of the data (inthis embodiment, the determined electromagnetic transfer functions)measured 704 by the receivers 104 may be used to determine the locationand shape of the anomaly. Three-dimensional (3D) forward and inverseelectromagnetic modeling may be used to make this determination.

FIG. 8 is a flow diagram of an embodiment of a method 800 for measuringsea-bed resistivity. The method 800 may include generating 802 anelectromagnetic field over a plurality of frequencies. For example, thefrequency domain current in the electromagnetic transmitter 102 maygenerate an electromagnetic field for at least several frequencies. Atleast one component of the electromagnetic field may be measured 804over the plurality of frequencies. For example, the receivers 104 maymeasure 804 the signal at several frequencies.

A conductivity distribution may be determined 806 based on the at leastone measured component. For example, the multi-frequency measurementsmay be used for frequency electromagnetic sounding of the medium atdifferent distances from the sea-bottom to produce a volume image of theconductivity distribution. The determined conductivity distribution maybe correlated 808 with geological formations and/or hydrocarbon deposits110.

FIG. 9 is a flow diagram of an embodiment of a method 900 for measuringsea-bed resistivity. The method 900 may include generating 902 anelectromagnetic field in the time domain. For example, the pulse (timedomain) current in the transmitter may generate 902 an electromagneticfield. At least one component of the electromagnetic field may bemeasured 904 in the time domain. For example, the receivers 104 maymeasure 904 the signal at different time moments.

A conductivity distribution may be determined 906 based on the at leastone measured component. For example, the transient measurements may beused for time domain electromagnetic sounding of the medium at differentdistances from the sea-bottom to produce a volume image of theconductivity distribution. The determined conductivity distribution maybe correlated 908 with geological formations and/or hydrocarbon deposits110.

FIG. 10 is a flow diagram of an embodiment of a method 1000 formeasuring sea-bed resistivity. The method 1000 may include generating1002 an electromagnetic field. The electromagnetic field may begenerated 1002 by an electromagnetic transmitter 102. Theelectromagnetic transmitter 102 may be located on land 314, like theelectromagnetic transmitter 302 described in FIGS. 3 and 5, orunderwater, like the electromagnetic transmitter 402 described in FIGS.4 and 6.

At least one component of the electromagnetic field may be measured1004. Receivers 104 may measure 1004 the electromagnetic field. Forexample, stationary and/or moving receivers 304, 504 may measure 1004 atleast one component of the electromagnetic field. In some embodiments,the amplitude and/or phase of the electromagnetic field may be measured1004. In further embodiments, the magnetic and/or electric components ofthe electromagnetic field may be measured 1004.

A conductivity distribution may be determined 1006 based on the at leastone measured component. The conductivity distribution may include theconductivity distribution for a hydrocarbon deposit 110, geologicalformations, and/or other formations and/or deposits. In someembodiments, the conductivity distribution may be determined 1006 bydetermining 706 at least one transfer function based on a plurality ofmeasured components of the electromagnetic field.

The determined conductivity distribution may be correlated 1008 withgeological formations and/or hydrocarbon deposits 110. In someembodiments, the determined at least one transfer function may becorrelated 708 with geological formations and/or hydrocarbon deposits110.

Observed data may be compared 1010 with predicted data. Traditionally,the electromagnetic inversion may be based on minimization of theparametric functional, P^(α)(Δσ) with the corresponding stabilizer s(Δσ)P ^(α)(Δσ)=φ(Δσ)+αs(Δσ)  (13)

where φ(Δσ) may represent the misfit functional between the predicteddata and the observed (i.e. measured 1004) data, and α is aregularization parameter.

The misfit functional may indicate how well the data predicted for agiven conductivity model fit with the observed data. The stabilizingfunctional (the stabilizer) may be used to bring the a prioriinformation about the desirable properties of the geological sectioninto the inversion algorithm. New stabilizers may be used, which maymake it possible to produce clearer and more focused images of theinverse models than the traditional maximum smoothness stabilizers. Forexample, minimum support (MS) and minimum gradient support (MGS)functionals may be useful in the solution of geophysical inverseproblems. These functionals may help to select the desired stablesolution from the class of solutions with the specific physical and/orgeometrical properties. In imaging a sea-bed hydrocarbon deposit 110using the ULF/ELF signal, one of these properties may include theexistence of sharp boundaries separating geological formations withdifferent physical parameters, e.g., oil and water saturated deposits inpetroleum exploration. This approach is typically called the regularizedfocusing inversion.

The parametric functional P (Δσ) may be minimized by using gradient typemethods. For example, the regularized conjugate gradient (RCG) algorithmof the parametric functional minimization in the case of the minimumnorm stabilizer may be summarized as follows:r _(n) =A(Δσ_(n))−d,(α)I _(n) =I(Δσ_(n))=ReF* _(n) W _(d) *W _(d) r _(n) +αW _(m) *W_(d)(Δσ_(n)−Δσ_(apr))(b)β_(n) =∥I _(n)∥² /∥I _(n−1)∥² ,Ĩ _(n) −I _(n)+β_(n) Ĩ _(n−1) ,Ĩ ₀ =I₀(c)k _(n)=(Ĩ_(n) ,I)/{∥W _(d) F _(n) Ĩ _(n)∥² +∥W _(m) Ĩ _(n)∥²},(d)Δσ_(n+1) =Δσ−k _(n) Ĩ _(n),(e)  (14)

where k_(n) may represent a length of the iteration step, and Ĩ_(n) mayrepresent the gradient direction, which may be computed using theadjoint Frechet derivative matrix, F*_(n), for the forward modelingoperator (12).

We may determine the data weights as a diagonal matrix formed by theinverse absolute values of the background field. Computation of themodel weighting matrix may be based on sensitivity analysis. We mayselect W_(m) as the square root of the sensitivity matrix in the initialmodel:W _(m)=√{square root over (diag(F ₀ *F ₀)^(1/2))}.  (15)

As a result, we may obtain a uniform sensitivity of the data todifferent model parameters.

By solving the electromagnetic inverse problem (12) we may produce a 3Dconductivity distribution in the sea-bottom geological formations. Theconductivity model may be inferred by inversion from the observed USBEMsurvey data that produces a 3D image of a sea-bottom hydrocarbon deposit110, associated with the high resistivity zone.

In the present embodiment, the measured at least one component of theelectromagnetic field may be stacked 1012 with a corresponding at leastone component of the electromagnetic field measured at another period.

Improvements to the signal-to-noise ratio may be realized by stacking1012 the observed signal over an appropriate period of time. In the caseof the uncorrelated noise, the signal-to-noise ratio may increase by√{square root over (N)} where N is the number of stacked signals. Forexample, if the frequency of the observed signal is about 0.1 Hz, it maybe sufficient to record this signal repeatedly over a two hour period toimprove a signal-to-noise ratio about 25 times.

FIG. 11 illustrates various components that may be utilized in acomputing device 1101. A receiver 104, an electromagnetic transmitter102 and/or other devices may be examples of a computing device 1101. Theillustrated components may be located within the same physical structureor in separate housings or structures.

The computing device 1101 may include a processor 1103 and memory 1105.The processor 1103 may control the operation of the computing device1101 and may be embodied as a microprocessor, a microcontroller, adigital signal processor (DSP) or other device known in the art. Theprocessor 1103 typically performs logical and arithmetic operationsbased on program instructions stored within the memory 1105. Theinstructions in the memory 1105 may be executable to implement themethods described herein.

The computing device 1101 may also include one or more communicationinterfaces 1107 for communicating with other electronic devices. Thecommunication interface(s) 1107 may be based on wired communicationtechnology, wireless communication technology, and/or othercommunication technology.

The computing device 1101 may also include one or more input devices1109 and one or more output devices 1111. The input devices 1109 andoutput devices 1111 may facilitate user input. Examples of differentkinds of input devices 1109 may include a keyboard, mouse, microphone,remote control device, button, joystick, trackball, touchpad, lightpen,etc. Examples of different kinds of output devices 1111 may include aspeaker, printer, etc. One specific type of output device which may beused in a computer system is a display device 1113. Display devices 1113used with embodiments disclosed herein may utilize any suitable imageprojection technology, such as a cathode ray tube (CRT), liquid crystaldisplay (LCD), light-emitting diode (LED), gas plasma,electroluminescence, or the like. A display controller 1115 may also beprovided, for converting data stored in the memory 1105 into text,graphics, and/or moving images (as appropriate) shown on the displaydevice 1113. Other components may also be provided as part of thecomputing device 1101.

FIG. 11 illustrates only one possible configuration of a computingdevice 1101. Various other architectures and components may be utilized.

Information and signals may be represented using any of a variety ofdifferent technologies and techniques. For example, data, instructions,commands, information, signals, bits, symbols and chips that may bereferenced throughout the above description may be represented byvoltages, currents, electromagnetic waves, magnetic fields or particles,optical fields or particles, or any combination thereof.

The various illustrative logical blocks, modules, circuits, andalgorithm steps described in connection with the embodiments disclosedherein may be implemented as electronic hardware, computer software orcombinations of both. To clearly illustrate this interchangeability ofhardware and software, various illustrative components, blocks, modules,circuits and steps have been described above generally in terms of theirfunctionality. Whether such functionality is implemented as hardware orsoftware depends upon the particular application and design constraintsimposed on the overall system. Skilled artisans may implement thedescribed functionality in varying ways for each particular application,but such implementation decisions should not be interpreted as causing adeparture from the scope of the present invention.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a general purpose processor, a digital signalprocessor (DSP), an application specific integrated circuit (ASIC), afield programmable gate array signal (FPGA) or other programmable logicdevice, discrete gate or transistor logic, discrete hardware components,or any combination thereof designed to perform the functions describedherein. A general purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

Functions such as executing, processing, performing, running,determining, notifying, sending, receiving, storing, requesting and/orother functions may include performing the function using a web service.Web services may include software systems designed to supportinteroperable machine-to-machine interaction over a computer network,such as the Internet. Web services may include various protocols andstandards that may be used to exchange data between applications orsystems. For example, the web services may include messagingspecifications, security specifications, reliable messagingspecifications, transaction specifications, metadata specifications, XMLspecifications, management specifications, and/or business processspecifications. Commonly used specifications like SOAP, WSDL, XML,and/or other specifications may be used.

The steps of a method or algorithm described in connection with theembodiments disclosed herein may be embodied directly in hardware, in asoftware module executed by a processor, or in a combination of the two.A software module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such that theprocessor can read information from, and write information to, thestorage medium. In the alternative, the storage medium may be integralto the processor. The processor and the storage medium may reside in anASIC. The ASIC may reside in a user terminal. In the alternative, theprocessor and the storage medium may reside as discrete components in auser terminal.

The methods disclosed herein comprise one or more steps or actions forachieving the described method. The method steps and/or actions may beinterchanged with one another without departing from the scope of thepresent invention. In other words, unless a specific order of steps oractions is required for proper operation of the embodiment, the orderand/or use of specific steps and/or actions may be modified withoutdeparting from the scope of the present invention.

While specific embodiments and applications of the present inventionhave been illustrated and described, it is to be understood that theinvention is not limited to the precise configuration and componentsdisclosed herein. Various modifications, changes and variations whichwill be apparent to those skilled in the art may be made in thearrangement, operation, and details of the methods and systems of thepresent invention disclosed herein without departing from the spirit andscope of the invention.

1. A method for measuring the resistivity of sea-bed formations comprising: generating an electromagnetic field using at least one stationary long-range transmitter, wherein the electromagnetic field is generated by a transmitter formed by a system of grounded electric bipoles of more than three kilometers in length or formed by a loop of wire with the radius of more than three kilometers, wherein the frequency of the electromagnetic field is between and/or including the ULF/ELF range; measuring at least one component of the electromagnetic field with a plurality of receivers, wherein the plurality of receivers are remotely located from the at least one stationary long-range transmitter; determining a conductivity distribution based on the at least one measured component; and correlating the determined conductivity distribution with geological formations and/or hydrocarbon deposits.
 2. The method of claim 1, wherein generating an electromagnetic field further comprises generating the electromagnetic field over a plurality of frequencies.
 3. The method of claim 2, wherein measuring at least one component of the electromagnetic field further comprises using a receiver that measures the electromagnetic field over a plurality of frequencies.
 4. The method of claim 1, wherein generating an electromagnetic field further comprises generating the electromagnetic field in the time domain.
 5. The method of claim 4, wherein generating the electromagnetic field in the time domain further comprises using a magnetohydrodynamic (MHD) generator.
 6. The method of claim 5, wherein measuring at least one component of the electromagnetic field further comprises using a receiver that measures the electromagnetic field in the time domain.
 7. The method of claim 1, wherein generating an electromagnetic field further comprises locating a transmitter on land or underwater.
 8. The method of claim 7, wherein the transmitter is an undersea communication cable.
 9. The method of claim 1, wherein measuring at least one component of the electromagnetic field further comprises using at least one galvanic receiver and/or at least one induction receiver.
 10. The method of claim 9, further comprising recording the voltage detected in at least one of the receivers.
 11. The method of claim 1, wherein measuring at least one component of the electromagnetic field further comprises measuring the amplitude and/or phase of electromagnetic field.
 12. The method of claim 1, wherein measuring at least one component of the electromagnetic field further comprises using at least one receiver located on a sea-bed and/or at least one receiver located in a sea-bed borehole.
 13. The method of claim 1, wherein measuring at least one component of the electromagnetic field further comprises using at least one moving receiver located underwater.
 14. The method of claim 1, wherein measuring at least one component of the electromagnetic field further comprises using a receiver that measures a magnetic component and/or electric component of the electromagnetic field.
 15. The method of claim 1, wherein determining a conductivity distribution is based on a plurality of measured components of the electromagnetic field and wherein the conductivity distribution is determined by determining at least one transfer function using the plurality of measured components of the electromagnetic field.
 16. The method of claim 15, wherein determining at least one transfer function further comprises determining at least one of the following transfer functions selected from the group consisting of: an electric transfer function, a magnetic transfer function, an impedance transfer function, and an admittance transfer function.
 17. The method of claim 15, wherein determining at least one transfer function further comprises using a linear relationship between a first component of the electromagnetic field and a second component of the electromagnetic field.
 18. The method of claim 15, wherein determining at least one transfer function further comprises using a least-squares method.
 19. The method of claim 1, wherein correlating the determined conductivity distribution with geological formations and/or hydrocarbon deposits further comprises characterizing sea-bed material, remotely detecting sea-bed hydrocarbon deposits, and/or imaging the sea-bed hydrocarbon deposits.
 20. The method of claim 1, wherein correlating the determined conductivity distribution with geological formations and/or hydrocarbon deposits further comprises determining a three-dimensional conductivity distribution.
 21. The method of claim 20, wherein determining a three-dimensional conductivity distribution further comprises using a three-dimensional inversion technique.
 22. The method of claim 21, wherein the three-dimensional inversion technique is based on a regularized three-dimensional focusing nonlinear inversion of the at least one measured component of the electromagnetic field.
 23. The method of claim 1, wherein correlating the determined conductivity distribution with geological formations and/or hydrocarbon deposits further comprises comparing observed data with predicted data.
 24. The method of claim 23, wherein comparing observed data with predicted data further comprises minimizing a parametric functional.
 25. The method of claim 24, wherein minimizing a parametric functional further comprises using gradient type methods and/or a misfit functional and a stabilizer.
 26. The method of claim 1, wherein correlating the determined conductivity distribution with geological formations and/or hydrocarbon deposits further comprises stacking the measured at least one component of the electromagnetic field with a corresponding at least one component of the electromagnetic field measured at another period.
 27. A method for locating hydrocarbon deposits comprising: generating an electromagnetic field using at least one remote stationary long-range transmitter, wherein the frequency of the electromagnetic field is between and/or including the ULF/ELF range, the electromagnetic field propogating to a plurality of receivers through an ionospheric waveguide and/or underground waveguide; measuring at least one component of the electromagnetic field using the plurality of receivers, wherein the plurality of receivers are located remotely from the at least one stationary long-range transmitter; determining at least one transfer function based on the at least one measured component, wherein the at least one transfer function is independent of a location of the at least one stationary long-range transmitter; and correlating the determined transfer function with geological formations and/or hydrocarbon deposits.
 28. A method for locating hydrocarbon deposits comprising: generating an electromagnetic field using at least one undersea communication cable carrying a frequency domain current, wherein the frequency of the electromagnetic field is within the ELF range; measuring a plurality of components of the electromagnetic field using a plurality of magnetic and/or electric receivers that are located at the sea-bottom, wherein the plurality of receivers measure the electromagnetic field over a plurality of frequencies, wherein the plurality of magnetic and/or electric receivers are located remotely of the at least one undersea communication cable; determining at least one transfer function based on the plurality of measured components, wherein at least one of the following transfer functions is selected from the group consisting of: an electric transfer function, a magnetic transfer function, an impedance transfer function, and an admittance transfer function is determined; correlating the determined at least one transfer function with geological formations and/or hydrocarbon deposits by determining a three-dimensional conductivity distribution using a three-dimensional inversion technique based on a regularized three-dimensional focusing nonlinear inversion of the plurality of measured components of the electromagnetic field; comparing observed data with predicted data by minimizing a parametric functional using gradient type methods and/or a misfit functional and a stabilizer; and stacking the plurality of measured components of the electromagnetic field with a corresponding plurality of measured components of the electromagnetic field measured at another period. 